1. Field of the Invention
This invention relates to the production of liquid fuels from coal and natural gas and, more specifically, to a method to eliminate carbon dioxide emissions and avoid the usage of water during the production of said fuels.
2. Description of the Related Art
A method to eliminate all carbon dioxide (CO2) emissions during the production of liquid fuels from coal and natural gas has been developed. Gasification of coal or coal-biomass mixtures produces synthesis gas (syngas), a mixture of hydrogen (H2) and carbon monoxide (CO) with H2/CO ratios in the range of approximately 0.5 to 0.9. Production of liquid fuels by Fischer-Tropsch synthesis (FTS) requires syngas with H2/CO ratios ≧2.0. Traditionally, the additional H2 required has been produced by the water-gas shift (WGS) reaction, which is shown below.H2O+CO→H2+CO2  (1)Unfortunately, the WGS reaction produces one molecule of CO2 and uses one molecule of H2O for each molecule of H2 it produces. There is currently a great deal of concern about the production of CO2, which is a green house gas, and a desire to eliminate or reduce the production of CO2 as much as possible.
For a large FTS plant, the WGS reaction produces huge amounts of CO2 and uses very large amounts of H2O. This patent demonstrates that the required H2 can be produced with no production of CO2 and no usage of H2O by combining Fischer-Tropsch synthesis (FTS) with catalytic dehydrogenation (CDH) of: (1. the gaseous hydrocarbons produced by FTS; and 2. additional methane injection (MI) into the CDH reactor.) This combined process will henceforth be designated as (FTS-CDH-MI). The additional methane for the (FTS-CDH-MI) process can be derived from natural gas that is currently being produced very economically by hydro-fracking. Separation of methane from the other components of natural gas such as CO2 can be achieved by pressure-swing adsorption.
The (FTS-CDH-MI) process eliminates the need for the water-gas shift (WGS) reaction, which has traditionally been used to increase the H2 content of the syngas used in FTS. This eliminates the production of CO2 and the use of water while producing the H2 needed for FTS of liquid fuels. As a bonus, the CDH reaction in the FTS-CDH-MI process produces a valuable by-product, multi-walled carbon nanotubes (MWCNT), which have many energy-related uses, as discussed below.
This research has demonstrated that MWCNT make excellent catalyst supports. They can be used to support many types of catalysts, including catalysts for FTS, CDH of hydrocarbon gases, and CDH of liquid hydrocarbons. Due to the fact that the MWCNT are normally not more than 7-10 nm in diameter, catalyst particles formed on the MWCNT supports by precipitation methods are typically about 1-3 nm in diameter. We have used Co-based catalysts supported on MWCNT for FTS and Fe-alloy catalysts (Fe—Ni, Fe—Mn, Fe—Pd and Fe—Mo) supported on MWCNT for CDH of methane, ethane, and propane. The Fe—Ni, Fe—Mn, and Fe—Pd catalysts all have a face-centered cubic (fcc) {fcc—austenitic} structure, while the Fe—Mo structure is somewhat more complicated. All four catalysts exhibit high activity for CDH of hydrocarbon gases. The Fe—Ni and Fe—Mn catalysts are also attractive because of the relatively low cost of Fe, Ni, and Mn. It is interesting to note that Fe—Mn alloys, which have the widest {(fcc)/austenitic} phase range of all the Fe-based alloy catalysts tested (33 to 53 at. % Mn at 300° C.), also exhibit the slowest rate of catalyst de-activation for CDH of methane in a quartz tube plug-flow reactor that has been observed to date (approximately 2% per hour).
In a somewhat different application, it was demonstrated that Pt nanoparticles 1-3 nm in diameter supported on stacked-cone nanotubes (SCNT) are excellent catalysts for the CDH of high-hydrogen content liquid hydrocarbons, such as cyclohexane and methyl-cyclohexane. The SCNT are produced by CDH of ethane, propane, and methane in the relatively low temperature range of 450 to 500° C. These catalysts are also active for the CDH of decalin and tetralin. CDH of liquid hydrocarbons could be valuable for the development of cars, trucks, and other vehicles powered by hydrogen fuel cells. The CDH of cyclohexane produces pure H2 and benzene, which is considered to be a pollutant. However, it is found that CDH of methyl-cyclohexane produces pure H2 and toluene, a valuable chemical. Toluene is an aromatic hydrocarbon that is widely used as an industrial feedstock and as a solvent. It is an important organic solvent, but it is also capable of dissolving a number of inorganic chemicals such as sulfur, iodine, bromine, phosphorus, and other non-polar covalent substances.
Synthesis gas (syngas) is a mixture of hydrogen (H2) and carbon monoxide (CO) produced by gasification of coal or mixtures of coal+biomass in oxygen and steam at high temperatures and pressures. Typically, coal constitutes 80-100% of the gasification feedstock. The Fischer-Tropsch synthesis (FTS) catalytically converts such syngas into liquid fuels and smaller amounts of hydrocarbon gases (methane, ethane, propane, etc.). The primary products of FTS are normally clean, high quality transportation fuels, including gasoline, jet fuel, and diesel fuel. The synthetic fuels resulting from the FTS process advantageously increase energy diversity. They also burn very cleanly and thus hold the promise of additional improved environmental performance, particularly for decreased emissions of fine airborne particulate matter, a topic of significant environmental concern. (“Speciation of Elements in NIST Particulate Matter SRMs 1648 and 1650,” F. E. Huggins and G. P. Huffman, 1999, J of Hazardous Materials, 74, 1-23); (“XAFS Spectroscopic Characterization of Elements in Combustion Ash and Fine Particulate Matter,” F. E. Huggins, N. Shah, G. P. Huffman, and J. D. Robertson, 2000, Fuel Proc. Tech., 65-66, 203-218 [Special Issue on Air Quality: Mercury, Trace Elements and Particulate Matter].)
Two previous patents by the applicant address the progression of methods for the reduction of CO2 during the production of liquid fuels from syngas derived from coal and natural gas (U.S. Pat. No. 8,268,897 and U.S. Pat. No. 8,309,616). The teachings and disclosures of both patents are incorporated herein by reference.
Two publications by the applicant and his collaborators demonstrate that CDH of ethane and propane produces a large amount of methane and small amounts of H2, multi-walled carbon nanotubes (MWCNT), and amorphous carbon. (N. Shah, D. Panjala, and G. P. Huffman, “Hydrogen Production by Catalytic Decomposition of Methane”, Energy & Fuels, 15 (2001) 1528-1534); and (N. Shah, D. Panjala, and G. P. Huffman, “Production of pure hydrogen and carbon nanostructures by catalytic non-oxidative dehydrogenation of ethane and propane”, Energy & Fuels, 18 (2004), 727-736).
A patent covering this material (U.S. Pat. No. 6,875,417) was obtained by the applicant and his co-inventors. The teachings of this patent and these publications are fully incorporated herein by reference.
A recent publication by the applicant demonstrates that a significant amount of the H2 required by FTS to produce liquid fuels can be produced by catalytic dehydrogenation (CDH) of the hydrocarbon gases produced by FTS. (G. P. Huffman, “Incorporation of catalytic dehydrogenation into Fischer-Tropsch synthesis of liquid fuels from coal to minimize carbon dioxide emissions”, Fuel, 90 (2011) 2671-2676). A patent covering this material was awarded to the applicant. (U.S. Pat. No. 8,268,897). The teachings of the patent are fully incorporated herein by reference.
The hydrocarbon gases produced by FTS include both paraffins (CnH(2n+2)) and olefins (CnH2n), with n=1 to 4, commonly referred to as (C1-C4) gases. (Irving Wender, “Reactions of Synthesis Gas, SPECIAL ISSUE, Fuel Processing Technology 48 (1996) 189-297). During the CDH reaction, the C2-C4 gases are quickly converted into methane and small amounts of amorphous carbon and H2 at temperatures of approximately 400 to 500° C. The dominant reaction above 500° C. is the CDH of methane;
                              CH          4                ↔                              [                          CH              4              ‡                        ]                    →                                    C              M                        +                          2              ⁢                                                          ⁢                              H                2                                                                        (        2        )            CM denotes carbon in the form of multi-walled carbon nanotubes (MWCNT). The CDH reaction is considered to be in a state of pseudo-equilibrium, denoted by the double arrow and the activated complex symbol, [CH‡4]. This concept was invented by Henry Eyring (Eyring, Henry, J. Chem. Phys. 3 (1935), 107-115). This reaction is obviously preferable to the WGS reaction (equation (1) on page 1) since it produces zero CO2 and uses no water. Its only products are H2 and MWCNT, a valuable by-product.
Formation of the MWCNT occurs because C atoms produced by CDH and deposited onto the catalyst surface at high temperatures diffuse into the catalyst. As the metallic catalyst becomes saturated with C, the carbon atoms precipitate out of the catalyst surface in the form of MWCNT, which is apparently the low energy structure. Typical results are shown in FIG. 1, where the H2/CO molar ratios produced by the FTS-CDH process are plotted as a function of (C1-C4) (wt. %) using experimental data from 26 different FTS data sets obtained from 10 different papers. These papers are referenced in a recent publication by the applicant (Gerald P. Huffman, “Incorporation of catalytic dehydrogenation into Fischer-Tropsch synthesis of liquid fuels from coal to minimize carbon dioxide emissions”, Fuel, 90 (2011) 2671-2676). Only FTS data obtained using cobalt-based FTS catalysts were used in obtaining the data shown in FIG. 1. No FTS data obtained using iron-based FTS catalysts were used because Fe is an excellent catalyst for the WGS reaction, which the current invention avoids in order to eliminate CO2 emissions and unnecessary usage of water.